The keys to a trim waistline and good health aren’t big secrets: Eat your vegetables, get some whole fruit, stick with lean protein, and go for whole grains.

Fiber, though, is one often overlooked part of the equation. (And the average Western diet is woefully lacking it.) But fiber might be a cornerstone of eating right and staying healthy, according to twonew studies on mice published in the journal Cell Host & Microbe.

In both investigations, researchers gave mice diets with hardly any fiber. The researchers then studied what happened in the rodents’ guts, and tracked changes in weight and blood sugar. Not surprisingly, all mice who ate little fiber eventually gained weight, suffered from high blood sugar, and developed insulin resistance. The mice in one study also showed that the protective layer of mucus in the colon became more porous, and allowed bad bacteria to breach it, setting up inflammation. The gut flora also exhibited changes with the beneficial bacteria dying off, which led to an unhealthy balance of microbiota in the gastrointestinal system.

Researchers were able to help restore some of the balance and health of the GI tract by adding fiber back into the diets of the mice, but it didn’t completely heal them, nor restore the diversity of original gut bacteria populations.

“Diets that lack fiber alter the bacterial composition and bacterial metabolism, which in turn causes defects to the inner mucus layer and allows bacteria to come close, something that triggers inflammation and ultimately metabolic disease,” said a study co-author Gunnar C. Hansson, a professor in the Mucin Biology Group at the University of Gothenburg, Sweden. “It’s not enough just to add fiber to your diet; it also depends on which bacteria you carry.”